1. Field of the Invention
The present invention relates to a filter device and, more particularly, to a distributed-constant-line-type filter.
2. Description of the Related Art
Since distributed-constant-line-type filters are typically formed of a strip line, they are thinner and lighter than filters using a block resonator, and are used for signal processing of a portable telephone set requiring a strictly smaller size.
FIG. 9 shows an example of a distributed-constant-line-type filter formed by combining conventional distributed-constant-line-type resonators. A distributed-constant-line-type filter 100 is a comb-line-type filter, namely, a comb-type filter formed of a plurality of distributed-constant-line-type resonators 101, 102, 103, 104 and 105, and a grounding electrode 106 connected to the resonators. The resonators are disposed at such positions as to be coupled to each other. One end of each of the distributed-constant-line-type resonators 101 to 105 is an open end. A distributed-constant line 107 for input is connected to the outermost resonator 101. A distributed-constant line 108 for output is connected to another outermost resonator 105. The distributed-constant-line-type resonators 101 to 105, the grounding electrode 106, the input/output distributed-constant lines 107 and 108 are strip lines or may be microstrip lines.
Since the distributed-constant-line-type resonators 101, 102, 103, 104 and 105 form a resonance circuit, components having a resonance frequency of a resonance circuit from among signal components from the input 107 are sent to the output 108, and other signals are reflected by the resonance circuit and return to the input 107. That is, the distributed-constant-line-type filter 100 operates as a band-pass filter.
As a main factor for a signal loss in the distributed-constant line, there is a loss due to resistance (line resistance). In order to reduce loss, it is common practice to widen the line width so as to lower the line resistance. However, in the conventional filter 100, if the line width of the resonator is widened, the spacing between the adjacent lines becomes narrow, and the coupling between the resonators becomes too strong, causing the characteristics of the filter to vary. In order to reduce the loss of the line and adjust the coupling between the resonators to a predetermined level, the spacing between the adjacent lines may be widened. However, the size of the filter in the right-to-left direction in the figure increases.